Complement activation in septic baboons detected by neoepitope-specific assays for C3b/iC3b/C3c, C5a and the terminal C5b-9 complement complex (TCC)

SUMMARY We have investigated the cross-reactivity of various species in ncocpitope-specific methods for quantification of human complement aclivatlon products. In contrast to most other species examined, baboon showed a substantial cross-reactivity supporting a high degree of homology between human and bahoon complement. An assay for C3b, iC3b and C3c (MoAh bH6) showed moderately good reactivity, in contrast to a C3a assay which did not cross-react. Excellent reactivity was found for C5a using MoAbs C17/5 and G25/2. The reactivity of an established TCC assay (MoAb aEU to a C9 neoepitopc and polyclonal antibody to C5) was improved substantially by replacing the anti-C5 antibody with a new MoAb to C6 particularly selected on the basis of baboon cross-reactivity. Plasma samples from baboons receiving 2·5 · 109 and 1·0 ± 1·010 live Escherichia coli bacteria/kg were examined with the assays described. In vivo complement aetivation with the lowest dose was moderate and kept under control, in contrast to the highest dose, where an uncontrolled increase in all activation products continued throughout the infusion period. These results support the hypothesis that sufficiently high amounts of endotoxin lead to uncontrolled activation of complement as seen in irreversible septic shock. The results arc discussed with particular emphasis on activation of the terminal complement pathway.